I'm sorry to say this, but some people in the Erlang community are wrong. Neither CouchDB, nor Ejabberd, nor SimpleDB, and not even Facebook Chat are killer apps for Erlang. Great examples of how powerful Erlang is? Yes. Need more of them? Yes, definitely. Killer apps in the way that Rails was for Ruby? No way.

Yes, apps like CouchDB and Ejabberd will increase the install base of Erlang as they gain in popularity. They'll drive a handful of curious programmers to give Erlang a close look. But it's very unlikely that they'll lead to the kind of massive uptake of Erlang like Rails did for Ruby.

Right now, the “killer apps” for Erlang would be more books. More documentation, especially tutorials aimed at newbies (to OTP/Erlang, not programming). Erlware is an extremely important project for Erlang's uptake – programmers have been “spoiled” by CPAN and RubyGems. The ultimate killer app for Erlang though, is something that'll give people a solid framework to quickly build something cool & sexy, in Erlang. I don't know what that is, although I do know that “concurrency” or “multicore programming” are not the killer apps either. Counting on those is a cop-out.

Perhaps, all that's needed is to rename OTP to Erlang On Rails, and to appoint an arrogant, brilliant Dane to take care of everything.